1. Field of the Invention
The present invention relates to an improvement in an optical pickup used in an optical recording and reproducing apparatus for recording and reproducing of an optical recording medium such as a magneto-optical disk. More specifically, the present invention relates to an improvement in an optical pickup integrated with optical system.
2. Description of the Background Art
When a signal recorded on a magneto-optical disk is reproduced by using an optical pickup, a beam of linearly polarized light is directed to the magneto-optical disk by the optical pickup. The plane of polarization of the beam reflected from the magneto-optical disk is rotated to right or left slightly dependent on magnetic direction on the magneto-optical disk by Kerr effect. With this reflected beam being passed through an analyzer, the direction of rotation of plane of polarization of the beam is detected as a variation in the amount of light, and recorded signal is reproduced.
FIG. 9 is a plan view of an optical pickup employing an optical waveguide device for a magneto-optical disk disclosed in Japanese Patent Laying-Open No. 8-171747. FIGS. 10 and 11 are a side view and a plan view respectively, of the optical waveguide device shown in FIG. 9.
When a signal recorded on the magneto-optical disk is to be detected (reproduced), the optical pickup generally directs a beam emitted from a laser diode to the magneto-optical disk. The beam is reflected and splitted into a beam for detecting a servo error signal and a beam for detecting the recorded signal, and the splitted beams are used by the optical pickup for detecting signals. At this time, a beam splitter is used for splitting the beam.
In the optical pickup PC1 shown in FIG. 9, a beam 102 emitted from laser diode 101 provided in a package 118 is divided into a main beam and a tracking beam by a grating 103 in package 118, passed through a hologram 104 in package 118, and incident on a beam splitter 105 formed by adhering a plate glass 113 and a prism 114. The beam entering beam splitter 105 is reflected by a mirror at an interface a between plane glass 113 and prism 114, passes through a collimator lens 106, reflected vertically upward by a mirror 107, and collected onto the magneto-optical disk (not shown) by an objective lens 108.
Thereafter, the beam reflected from the magneto-optical disk passes through objective lens 108, mirror 107 and collimator lens 106 and enters beam splitter 105, where the beam is splitted into a beam 109 for detecting a servo error signal, and a beam 110 for detecting the recorded signal. Beam 109 enters from beam splitter 105 to hologram 104, where the beam is diffracted, and thereafter the beam enters a photodiode 111 and detected by photodiode 111 as a servo error signal. Beam 110 is reflected by a mirror surface on a rear surface b of plane glass 113 which constitutes beam splitter 105, and therefore, it does not pass through hologram 104 but enters a coupler portion of optical waveguide device 112. Beam 110 which is coupled to the optical waveguide at this coupler portion is divided into TE beam and TM beam, and enter a photodetector, where the beams are detected (reproduced) as the information signal.
Referring to FIGS. 10 and 11, the coupler portion of optical waveguide device 112 will be described. The coupler portion includes a prism 121 and a microlens 122. Beam 110 reflected at the surface b of beam splitter 105 passes the right side of hologram 104, enters package 118 and is once converged and thereafter diverged. Then, the beam passes through microlens 122 and enters prism 121. At this time, the diverged beam 110 is converted to a collimated beam by microlens 122 provided on prism 121, and the collimated beam is coupled to optical waveguide 123 at a prescribed incident angle. The beam coupled to optical waveguide 123 is divided into TE and TM beams by a polarized beam splitter 129, and detected (reproduced) as the information signal, by photodiode 124.
In optical pickup PC1 of FIG. 9, laser diode 101 and optical waveguide device 112 are attached to package 118 and, thereafter, beam splitter 105 is attached to package 118. Therefore, offset in the position of attachment of optical waveguide 112, or relative positional offset between the beam reflected from the magneto-optical disk and optical waveguide device 112 caused by error in manufacturing plate glass 113 of beam splitter 105 must be compensated for by position adjustment of beam splitter 105.
FIG. 12 shows a principle of compensation of the relative positional offset between the beam reflected from the magneto-optical disk and optical waveguide device 112 by adjusting attitude of beam splitter 105 shown in FIG. 9. Referring to FIG. 12, assume that optical waveguide device 112 is arranged offset in the direction of the arrow Y. At this time, the beam emitted from laser diode 101 proceeds along an optical path L101, is reflected by a surface a of prism 114, proceeds along an optical path L102 and is incident on the magneto-optical disk. Thereafter, the beam reflected from the magneto-optical disk proceeds along optical path L102, is reflected at surface b of plate glass 113 and proceeds along an optical path L103 to optical waveguide device 112. At this time, assume that relative position between optical path L103 and optical waveguide device 112 is offset. When beam splitter 105 is rotated by .theta. about the X axis, the beam reflected from the magneto-optical disk would proceed along optical paths L202.fwdarw.L203 denoted by the dotted lines, and correctly enter optical waveguide device 112.
In the optical pickup PC1 of FIG. 9, beam splitter 105 is arranged between collimator lens 106 and hologram 104, which means that it is at a considerable distance from the light source, and therefore it requires a large effective aperture (the scope through which the beam passes in beam splitter 105). As a result, beam 110 converges very close to a lower surface of a member 117 on which grating 103 is formed, and therefore the point of convergence cannot directly be coupled to optical waveguide 123. From this reason, microlens 122 for converting the divergent beam 110 to a collimated beam has been required. Focal distance of microlens 122 is about 1 mm. It is difficult to form a lens having such a short focal distance on prism 121 of the coupler.
Further, since the surface a is inclined when the attitude of beam splitter 105 is adjusted, optical path L101 of the beam emitted from laser diode 101 is offset from the original optical path L102 by 2.theta., to optical path L202. As a result, the center of the beam emitted from laser diode 101 may possibly be offset from the center of collimator lens 106, or the collimated beam emitted from collimator lens 106 may proceed obliquely. It has been difficult to work out and apply a solution to such problems.
Further, in the optical pickup PC1 shown in FIG. 9, in order that one main beam spot and two tracking beam spots have matched orientation on a track of the magneto-optical disk, a separate mechanism for rotating package 118 containing laser diode 101 and beam splitter 105 about an optical axis (see chain-dotted line CL in the figure) of collimator lens 106 has been required.
The beam emitted from laser diode 101, passed through surface a and reflected at surface b is reflected from the magneto-optical disk and is detected (reproduced) as a signal by a photodetector. In order to prevent deterioration of quality of the detected signal, it has been necessary to form an antireflection film 116 partially at a portion of surface b which opposes to laser diode 101. This lowers efficiency in mass production of beam splitter 105.
Further, since optical waveguide device 112 is directly arranged in package 118, three-dimensional positional adjustment including adjustment of height at the time of arrangement has been difficult.
Further, an optical pickup has been proposed which employs an optical system including a cylindrical lens combined with a coupler prism on the optical waveguide, in order to converge the beam reflected from the disk and to couple the beam with the optical waveguide. In this optical system, the number of components is increased because of the provision of the cylindrical lens. Therefore, this optical pickup is disadvantageous in that it has considerably large scale.